In next-generation wireless communication systems, there are growing concerns about the exhaustion of frequency resources with the demand for wider bandwidths of transmission rates and the growing diversity of systems. Recently, cognitive radio for recognizing a surrounding radiowave environment and the needs of users to perform optimum communication autonomously has been considered. In this cognitive radio, dynamic spectrum access for allowing a frequency band allocated to an existing radio system to be secondarily used by another radio system has received attention from the standpoint of effective use of frequency resources. Specifically, dynamic spectrum access is to allow a secondary system as a new radio system to use a vacant spectrum in the frequency band allocated to a primary system as the existing radio system in such a manner not to interrupt communication of the primary system.
FIG. 27 is an explanatory drawing showing an example of a communication system for dynamic spectrum access. In the example shown in FIG. 27, a secondary system 2220 uses a vacant spectrum in a frequency band allocated to a primary system 2210 in such a manner not to interrupt communication of the primary system. In other words, the uplink or downlink of the secondary system 2220 shares the frequency band allocated to the uplink or downlink of the primary system 2210.
In the communication system shown in FIG. 27, the primary system 2210 includes a primary system base station 2211, a primary system mobile station 2212, and a primary system mobile station 2213. The primary system base station 2211 allows the exchange of data between the primary system mobile station 2212 and the primary system mobile station 2213.
Further, the secondary system 2220 includes a secondary system base station 2221, a secondary system mobile station 2222, and a secondary system mobile station 2223. The secondary system base station 2221 allows the exchange of data between the secondary system mobile station 2222 and the secondary system mobile station 2223.
As an example of dynamic spectrum access other than the example shown in FIG. 27, there can be IEEE802.22 WRAN (Wireless Regional Area Network). IEEE802.22 is a standard in the United States for systems in which, in a frequency band for land-based television broadcasting or a wireless microphone as an existing primary system, a fixed wireless access system as a secondary system uses a free channel of the frequency band of the primary system.
Next, an interference suppression technique related to dynamic spectrum access will be described. FIG. 28 is an explanatory drawing showing examples of band frequencies used by the primary system and spectra of the secondary system. FIG. 28(a) is a schematic diagram showing the band frequencies of the primary system and images of the spectra of the secondary system before the application of the interference suppression technique. FIG. 28(b) is a schematic diagram showing the band frequencies of the primary system and images of the spectra of the secondary system after the application of the interference suppression technique. In FIG. 28, the abscissa represents frequency and the ordinate represents power density.
In principle, the secondary system needs to perform communication not to interrupt the communication of the primary system. Therefore, in the example shown in FIG. 28, the secondary system spectra 2302-1 and 2302-2 need to be transmitted by suppressing interference with the band frequencies 2301-1, 2301-2, and 2301-3 used by the primary system. However, as shown in FIG. 28(a), in the actual transmission spectra, there is leakage power leaking into the outside of the transmission band. Therefore, there is fear that part of the spectra of the secondary system will interfere with the primary system because of this leakage power. Here, if sufficient guard bands (vacant frequency bands for guard) are provided for the band frequencies used by the primary system, interference with the primary system can be suppressed. However, when the sufficient guard bands are provided, there is fear of a decrease in frequency use efficiency.
As mentioned above, in a communication system using a cognitive radio technique for sharing the same frequency band between two or more systems, it is important for a secondary system to provide transmission while suppressing interference with the primary system without reducing the frequency use efficiency. When the secondary system is a system using an OFDM (Orthogonal Frequency Division Multiplexing) based wireless access method, since leakage power into the outside of the band becomes large due to sidelobe components of a subcarrier, interference suppression measures are particularly important.
For example, interference suppression transmission methods for suppressing interference with the primary system can include a digital filter system, a null regeneration method, a Gaussian multicarrier system, subcarrier weighting, time windowing, and AIC (Active Interference Cancellation). The digital filter system is a method of shaping a spectrum by using an FIR (Finite Impluse Response) filter or an IIR (Infinite Impluse Response) filter. The null regeneration method is a method of performing an FFT (Fast Fourier Transform) after multiple OFDM symbols are combined and performing an IFFT (Inverse Fast Fourier Transform) after null subcarrier replacement. The Gaussian multicarrier system is a multicarrier transmission system for shaping a spectrum with a Gaussian pulse waveform. Subcarrier weighting is a method of suppressing interference by weighting between symbols to be converted to subcarrier signals. Time windowing is a method of shaping an OFDM symbol in a time domain. AIC is a method of generating a tone for canceling an out-of-band leakage component to suppress interference. Among them, the AIC method having a high affinity for commercially existing radio systems and capable of dynamically suppressing interference to fit the surrounding radiowave conditions will be described below.
FIG. 29 is an explanatory drawing showing an example of the AIC method described in Non Patent Literature (NPL) 1. FIG. 29 shows a state in which a secondary system transmits spectra 2502-1 and 2502-2 on both sides of band frequencies used by the primary system 2501, plotting frequency on the abscissa and power density on the ordinate. AIC is a method for providing dedicated tones (AIC tones) for canceling components of the secondary system spectra leaking into the band frequencies used by the primary system to suppress out-of-band power of the secondary system. In the example shown in FIG. 29, a total of two AIC tones 2503-1 and 2503-2 are provided, one for each side, outside of the band frequencies used by the primary system 2501 to suppress the out-of-band power of the secondary system. The use of AIC can lead to considerable suppression of the out-of-band power.
FIG. 30 is an explanatory drawing showing an example of a CC (Cancellation Carrier) method described in NPL 2 as a modification of AIC. FIG. 30 shows the relationships between a transmission band 2602 and frequency positions (interference avoidance band) 2601 outside the transmission band, plotting frequency on the abscissa and power density on the ordinate. Like AIC, the CC method provides dedicated tones (CC) for canceling spectrum components leaking from the transmission band 2602 into the interference avoidance band 2601. Note here that the term CC is a kind of AIC tone described in NPL 1 to be generated for suppressing power in a partial frequency band of the interference avoidance band, i.e., in a partial interference avoidance band. In the example shown in FIG. 30, among frequencies in the transmission band 2602, two CCs 2603 are provided at frequencies located near the interference avoidance band 2601. The CCs 2603 are operative to minimize the leakage power of the partial interference avoidance band located near the transmission band 2602 and having a large amount of power particularly leaking therein among the frequencies in the interference avoidance band.
Patent Literature (PTL) 1 also describes an example of interference suppression processing by AIC.